1 to 3 seconds:
The amygdala, in turn, overrides the prefrontal cortex--a.k.a. your brain's logic center--and tells your adrenal glands to shoot out the stress hormones adrenaline and cortisol. Those prompt sweating (to help you stay cool), rapid breathing and heartbeat (to help pump oxygen to your muscles), and dilation of the pupils (to help you better keep an eye on that threat).

Within 5 seconds:
Your brain begins releasing endorphins, natural painkillers that could protect you during a physical attack. It also secretes a small amount of the feel-good hormone dopamine (hence the "rush" some people get in scary situations)--but not enough to temper your hormone-fueled panic. Some phobics experience sharp rises and drops in blood pressure, causing them to pass out. Many experts believe this stems from an evolutionary "freeze" or "play dead" response.

During and after the next few minutes:
If you can't calm yourself down, you'll need to flee the scene to get your prefrontal cortex back online. Once the audible or visual threat is gone, it directs the amygdala to put a stop order on those stress hormones. You might still be jumpy for a day or two (every stick could be a snake). If yours is a true phobia, your amygdala will spur this same outsize response every time--until you can retrain your brain to respond to your phobic trigger in a more logical way.